1. Field of the Invention
The present invention relates to a reflective optical system used for projectors or projection televisions to modulate and project image bearing light as an enlarged image on a screen.
2. Description of Related Art
Projectors for projecting images on a remote wide screen are provided with a projection optical system comprises a large number of lenses. Such a projector is unavoidably large in size. Apertures, curvatures and weight of these lenses used in the projection optical system exert various restraints on proximity wide-angle projection. In recent years, there have been proposed reflective projectors for projecting images on a wide screen at a short distance. Such a reflective projector is provided with a wide-angle projection optical system including an image display device or element such as a liquid crystal display device or a digital mirror device (DMD). The projection optical system disclosed in, for example, Japanese Unexamined Patent Publication No. 2002-40326 includes a folded reflective optical system including a plurality of reflection mirrors for reflecting image bearing light from the image display device for repeating reflection of image bearing light for enlarged image projection on a wide screen at a comparatively short distance. Using the folded reflective optical system enables wide-angle projection at a short distance and makes the projector compact in size. Moreover, the folded reflective optical system provides high resolution images.
However, the reflective optical system including a plurality of reflection mirrors encounters a difficulty in arranging the reflection mirrors in precise relative positions. In the case of, for example, a folded reflective optical system for a projector disclosed in Japanese Unexamined Patent Publication No. 2003-29149 in which six reflection mirrors are used for the purpose of aberration correction, there is an essential demand for the reflective optical system that mirror parts have to be not only prepared with high precision but arranged and fixed with a high degree of accuracy when attempting to miniaturize the projector and/or to enable wide-angle projection. This demand results in a drop in manufacturing efficiency and a raise in manufacturing cost.
Further, as disclosed, for example, in Japanese Unexamined Patent Publication No. 2003-29149, the reflective optical system is installed by mounting reflection mirrors to a housing. In order to avoid an occurrence of aberration such as distortion due to an accumulated error of registration of the reflection mirrors, the reflection mirrors have to be installed and fixed with a high degree of accuracy and also have to be adjusted precisely in position after installation.